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Hindawi Publishing Corporation EURASIP Journal on Wireless Communications and Networking Volume 2006, Article ID 17202, Pages 1–3 DOI 10.1155/WCN/2006/17202 Editorial Multiuser Cooperative Diversity for Wireless Networks George K. Karagiannidis, 1 Chintha Tellambura, 2 Sayandev Mukherjee, 3 and Abraham O. Fapojuwo 4 1 Department of Electrical and Computer Engineering, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece 2 Department of Electrical and Computer Engineering, University of Alberta, Edmonton, AB, Canada T6G2V4 3 Marvell Semiconductor, Santa Clara, CA 95054, USA 4 Department of Electrical and Computer Engineering, The University of Calgary, 2500 University Drive N.W., Calgary, AB, Canada T2N1N4 Received 24 September 2006; Accepted 24 September 2006 Copyright © 2006 George K. Karagiannidis et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Multihop relaying technology is a promising solution for fu- ture cellular and a d hoc wireless communications systems in ordertoachievebroadercoverageandtomitigatewireless channels impairment without the need to use high power at the transmitter. Recently, a new concept that is being actively studied in multihop-augmented networks is multiuser coop- erative diversity, where several terminals form a kind of coali- tion to assist each other with the transmission of their mes- sages. In general, cooperative relaying systems have a source node multicasting a message to a number of cooperative re- lays, which in turn resend a processed version to the intended destination node. The destination node combines the signal received from the relays, possibly also taking into account the source’s original signal. Cooperative diversity exploits two fundamentals features of wireless medium: its broadcast na- ture and its ability to achieve diversity through independent channels. There are three advantages from this. (1) Diversity. This occurs because different paths are likely to fade independently. The impact of this is expected to be seen in the physical layer, in the design of a receiver that can exploit this diversity. (2) Beamforming gain. The use of directed beams should improve the capacity on the individual wireless links. The gains may be particularly significant if space-time coding schemes are used. (3) Interference mitig ation. A protocol that takes advan- tage of the wireless channel and the antennas and re- ceivers available could achieve a substantial gain in sys- tem throughput by optimizing the processing done in the cooperative relays and in the scheduling of retrans- missions by the relays so as to minimize mutual inter- ference and facilitate information transmission by co- operation. In response to the demand for novel ideas and results, this special issue presents a sample of current a ctivities and up-to-date efforts in design, implementation, and perfor- mance analysis of cooperative diversity s ystems. A brief sum- mary of each paper is listed as follows. In the first paper by Z. Yang and A. Høst-Madsen, the cooperation efficiency of the multiple-relay channel when carrier-level synchronization is not available is investigated, assuming that all nodes use a decode-forward scheme. It is shown that by using decode-forward relay signaling, the transmission is interference-free, even when all communica- tions share one common physical medium. Furthermore, for any channel realization, there always exist a sequential path and a corresponding simple power-allocation policy, which are optimal. To illustrate the efficiency of cooperation and provide prototypes for practical implementation of relay- channel signaling, the authors propose two heuristic algo- rithms. Finally, the numerical results show that in the low- rate regime, the gain from cooperation is limited, while the gain is considerable in the high-rate regime. In the second paper, by D. Wang and U. Tureli, the au- thors try to face the inefficiencies caused due to the exist- ing medium-access control (MAC) schemes, when multiple- input multiple-output (MIMO) transmit/receive schemes and orthogonal f requency-division multiplexing (OFDM) are used in broadband multihop ad hoc networks. A new transceiver architecture with MIMO-OFDM and MAC scheme is proposed, named multiple-antennas receiver- initiated busy-tone medium access (MARI-BTMA), which 2 EURASIP Journal on Wireless Communications and Networking is based on receiver-initiated busy-tone medium access (RI- BTMA) and uses multiple out-of-band busy tones to avoid the collision of nodes on the same channel. With the pro- posed MAC scheme, multiple users can transmit simulta- neously in the same neighborhood. Although the proposed MARI-BTMA shows good performance at high trafficload, to improve the performance at low traffic loads, 1-persistent MARI-BTMA is proposed so that users can choose differ- ent MAC schemes according to the statistical trafficloadin the system. In the same paper, both theoretical and numeri- cal analyses of the throughput and delay are presented, while analytical and simulation results show the improved perfor- mance of MARI-BTMA compared with RI-BTMA and car- rier sensing medium access/collision avoidance (CSMA/CA). In the third paper, by Y. Yuan et al., a cluster-based co- operative MIMO scheme is proposed to reduce the adverse impacts caused by radio irregularity and fading in multihop wireless sensor networks. This is an extension of the LEACH protocol, enabling the multihop transmissions among clus- ters by incorporating a cooperative MIMO scheme into hop- by-hop transmissions. The proposed scheme can gain effec- tive performance improvement through the adaptive selec- tion of cooperative nodes and the coordination between mul- tihop routing and cooperative MIMO transmissions. More- over, the optimal parameters which minimize the overall en- ergy consumption, such as the number of clusters and the number of cooperative nodes, are derived. Simulation results exhibit that the proposed scheme can effectively save energy and prolong the network lifetime. In the fourth paper, by T. Abe et al., the MIMO relay scheme is proposed where each of the multiple-relay nodes performs QR decompositions of the backward and forward channelmatricesinconjunctionwithphasecontrol(QR-P- QR). A group nulling approach is used to decompose a mul- tiple source-destination (SD) MIMO relay channel into par- allel independent SD MIMO relay channels, and then apply the QR-P-QR scheme to each of the decomposed MIMO re- lay links. Numerical examples show that the proposed re- lay scheme offers higher capacity than other existing relay schemes. In the last paper by T. A. Tsiftsis et al., the end-to- end performance of dual-hop cooperative selective diversity links, equipped with nonregenerative relays and operating over nonidentical Nakagami-m fading channels, is studied. Closed-form expressions are presented for the cumulative distribution function and the probability density function of the end-to-end sig nal-to-noise ratio (SNR), while analyti- cal formulae are derived for the moments and the moment- generating function. The proposed mathematical analysis is complemented by numerical examples, including the effects on the overall performance of the SNRs unbalancing as well as the fading severity. ACKNOWLEDGMENTS We would like to thank all the authors of the papers submit- ted (either accepted or not) for considering this issue as a means of publication of their own work, the reviewers who allowed us to make our editorial decisions in a timely man- ner, and the Editor-in-Chief, Phillip Regalia, for giving us the opportunity and the support to achieve this special issue. We hope that these five papers will contribute to the literature of this very exciting research area and also motivate further research. George K. Karagiannidis Chintha Tellambura Sayandev Mukherjee Abraham Fapojuwo George K. Karagiannidis wasborninPi- thagorion, Samos Island, Greece. He re- ceived his university degree in 1987 and his Ph.D. degree in 1999, both in elect rical en- gineering, from the University of Patras, Pa- tras, Greece. From 2000 to 2004, he was a Researcher at the Institute for Space Appli- cations and Remote Sensing, National Ob- servatory of Athens, Greece. In June 2004, he joined the faculty of Aristotle University of Thessaloniki, Greece, where he is currently an Assistant Profes- sor at the Electrical and Computer Engineering Department. His major research interests include wireless communications theory, digital communications over fading channels, cooperative diversity systems, satellite communications, and free-space optical commu- nications. He has published and presented more than 70 technical papers in scientific journals and international conferences, he is a coauthor in two chapters in books and also coauthor in a Greek edition book on mobile communications. He acts as a reviewer for several international journals and he served as a Technical Program Committee Member for ICC ’04 and ICC ’05. He is Member of the editorial boards of IEEE Communications Letters and EURASIP Journal on Wireless Communications and Networking. Chintha Tellambura received the B.S. de- gree (with first-class honors) from the Uni- versity of Moratuwa, Moratuwa, Sri Lanka, in 1986, the M.S. degree in electronics from the University of London, London, UK, in 1988, and the Ph.D. degree in electri- cal engineering from the University of Vic- toria, Victoria, BC, Canada, in 1993. He was a Postdoctoral Research Fellow with the University of Victoria (1993–1994) and the University of Bradford (1995–1996). He was with Monash Univer- sity, Melbourne, Australia, from 1997 to 2002. Presently, he is a Professor with the Department of Electrical and Computer Engi- neering, University of Alberta, Canada. His research interests in- clude coding, communication theory, modulation, equalization, and wireless communications. He is an Associate Editor for both the IEEE Transactions on Communications and the IEEE Trans- actions on Wireless Communications. He was Chair of the Com- munication Theory Symposium in Globecom ’05 held in St. Louis, Mo, USA. George K. Karagiannidis et al. 3 Sayandev Mukherjee was born in Banga- lore, India, in 1970. He received the Bach- elor of Technology degree in electrical en- gineering from the Indian Institute of Tech- nology, Kanpur, India, in 1991, and the M.S. and Ph.D. degrees in electrical engineering from Cornell University, Ithaca, NY, in 1994 and 1997, respectively. From 1996 to Oc- tober 2006, he was a member of technical staff in the Wireless Research Laboratory, Bell Labs, Lucent Technologies, NJ, USA. He is now a Senior Staff Design Engineer with Marvell Semiconductor Inc., Santa Clara, Calif, USA. His research interests include stochastic models, wire- less system simulations, and connectivity issues in ad hoc wireless networks. Abraham O. Fapojuwo received the B.Eng. degree (first-class honors) from the Univer- sity of Nigeria, Nsukka, in 1980, and the M.S. and Ph.D. degrees in electrical engi- neering from the University of Calgary, Cal- gary, AB, Canada, in 1986 and 1989, re- spectively. From 1990 to 1992, he was a Re- search Engineer with NovAtel Communica- tions Ltd., w here he performed numerous exploratory studies on the architectural def- inition and performance modeling of digital cellular systems and personal communications systems. From 1992 to 2001, he was with Nortel Networks, where he conducted, led, and directed system- level performance modeling and analysis of wireless communica- tion networks and systems. In January 2002, he joined the Depart- ment of Electrical and Computer Engineering, University of Cal- gary, as an Associate Professor. He is also an Adjunct Scientist at TRLabs, Calgary. His current research interests include protocol design and analysis for future generation wireless communication networks and systems, and best practices in software reliability en- gineering and requirements engineering. He is a registered profes- sional Engineer in the Province of Alberta. . Corporation EURASIP Journal on Wireless Communications and Networking Volume 2006, Article ID 17202, Pages 1–3 DOI 10.1155/WCN/2006/17202 Editorial Multiuser Cooperative Diversity for Wireless Networks George. multihop-augmented networks is multiuser coop- erative diversity, where several terminals form a kind of coali- tion to assist each other with the transmission of their mes- sages. In general, cooperative relaying. account the source’s original signal. Cooperative diversity exploits two fundamentals features of wireless medium: its broadcast na- ture and its ability to achieve diversity through independent channels.